1. Field of the Invention
The present invention relates to a microwave oscillator with improved phase noise characteristic near the carrier.
The invention has its preferred but nonlimiting application in telecommunications systems and in particular in digital radio link systems with low and medium capacity and therefore is illustrated and described in detail below with reference to this application but its employment is not to be understood as limited to that illustrated.
In low and medium capacity radio link systems it is constantly required to reduce the spectrum occupation even if this involves the use of more complicated modulation schemes.
It has been found that a low phase noise microwave oscillator is fundamental for this reduction, at this purpose a microwave multiloop frequency synthesizer is an optimal solution to obtain an oscillator like that.
But the circuitry complexity of a multiloop synthesizer is considerable because of the devices employed and among which are in particular frequency dividers, sampling phase detectors, phase comparators, crystal oscillators, etc.
2. Description of the Related Art
U.S. Pat. No. 5,036,299 to Dick et al. pertains to an apparatus and a method for reducing microwave oscillator output phase noise. The patent is herein incorporated by reference. With reference to FIG. 9 of the patent, an apparatus is disclosed for reducing noise in the output of a tunable microwave oscillator. The apparatus includes a feedback arrangement for providing a corrective signal. The feedback arrangement includes a resonator for receiving the output of the oscillator, and an active phase detector for detecting fluctuations in the output by comparing the output with the signal emitted by the resonator. The improvement in the prior art patent includes: means for suppressing the carrier of the signal emitted by the resonator, a radio-frequency amplifier for amplifying the suppressed carrier; the means and the amplifier are located between the resonator and the phase detector. The apparatus disclosed is added to a traditional tunable microwave oscillator based on a low-Q dielectric resonator (col. 4, lines 37 and 38) about the identical oscillator. The above-mentioned corrective signal obtained by the feedback arrangement is derived by a second high-Q dielectric resonator.
The main shortcoming of the cited invention is the excessive circuitry and the complexity thereof. That complexity is illustrated in FIG. 9 of the patent with the additional important consideration that two dielectric resonators are required.